Have you tried contacting Customer Support? The reason people are advising you to do this is because this is what you should do if you have a legitimate complaint.

Also, you haven’t specified why you want to report your seller. If it is a problem with the quality of work produced etc, you should post about this in ‘The Ranting Pot’ but in order to get any direct assistance regarding a refund or the like you really need to just contact customer support as suggested.

You just click on the text. You’ll see it’s colored in blue meaning it’s linked to the place where you can contact them. Anyway, I’ve provided with you a link you can copy paste to your browser or even simply click. Just fill that up and you can bring up your concerns to them. http://support.fiverr.com/hc/en-us/requests/new